Engine oil lubrication systems, which are typical of many fluid systems, frequently utilize a spin-on on type filter unit which is mounted on a filter pad and which includes a filter element to remove potentially damaging particles from the oil. Mechanical wear within the engine, the outside environment, and contaminants introduced accidentally during normal servicing are sources of particles which may plug lubricating nozzles, severely damage parts, and create excessive wear on any surfaces relying on a thin film of lubricating oil for protection.
These systems typically utilize a pump to force oil to circulate through the filter pad and spin-on filter unit assembly and then to the moving parts of the engine for lubrication. Oil is forced through the filter element due to a difference in oil pressure, i.e., a pressure differential, between the upstream and downstream sides of the filter element, the pressure on the upstream side being greater than the pressure on the downstream side.
Over time, as the spin-on filter unit performs its filtering function, the filter element of the spin-on filter unit becomes plugged by particles removed from the lubricating oil. As the amount of material filtered from the lubricating oil and retained by the filter element increases, a greater pressure differential across the filter element is required to pump sufficient fluid through the filter element. The amount of lubricating oil which will pass through the filter element at the maximum pumping pressure will decrease until flow is insufficient to maintain proper lubrication of the engine. To prevent that occurrence, many lubrication systems include a pressure-sensitive bypass valve which will open when the pressure differential across the filter element approaches that corresponding to the maximum pumping pressure, allowing oil to bypass the filter element. This assures that lubricating oil will continue to reach the moving parts of the engine but eliminates, at least in part, the filtering function.
Thus, to assure that sufficient filtered oil continues to reach the moving parts of the engine, filter units must be replaced before they become so clogged as to cause the bypass valve to open. As few lubrication systems are equipped to monitor the pressure differential across the filter element to determine when the filter unit should be replaced, the spin-on filter units are replaced at regular intervals of operation. Although the rates at which a filter element becomes blocked will vary greatly depending upon operating conditions for the engine, the filter unit replacement interval must be selected to be the shortest probable time in which excessive blockage may occur to assure continuous proper lubrication system function. This results in the frequent disposal of filter units before their filtering capacity has been fully utilized, resulting in higher operational costs.
Some lubrication systems utilizing spin-on filter units include a pressure detector which senses the pressure only on the high pressure side, i.e., the upstream side, of the filter. The detector may be used to operate a filter blockage warning light when the upstream pressure becomes greater than a predetermined amount, e.g., approaches maximum pumping pressure. However, the pressure on the upstream side of the filter element is dependent on the overall lubrication system pressure and cumulative pressure drop through the system and is not necessarily indicative of the filter condition. Thus, such a detector may result in a false indication of the need to replace a filter element if the system becomes blocked downstream of the filter unit.
Few new engines have lubrication systems equipped with a pressure differential monitor because of the expense of providing instrumentation to measure pressure on both the upstream and downstream sides of the filter element and determine the difference between the two. The difficulty of accommodating such a monitoring system within the confines of the lubrication systems is also an important factor. Retrofit of existing engines to provide for monitoring of the differential pressure across the filter element of the lubrication system is particularly difficult if pressure-sensing devices must be installed within the crankcase together with leads for appropriate instrumentation.